Natural gas, petroleum products, and chemicals are pumped through pipelines. The pipeline is typically formed of a high grade or quality of steel. The pipeline is buried in the earth. As a result of the flow of the product through the pipeline, galvanic currents flow in the earth from the pipeline. These currents tend to weaken the steel pipeline by depositing some of the metal of the pipeline in the earth. The situation, if not prevented, can become so bad that the wall of the pipe is weakened and catastrophic ruptures can occur. Indeed, substantially all the metal can be carried away and a hole formed in the pipe. This is dangerous and is often the cause of explosions.
Pipelines have heretofore been protected by the incorporation of an insulated material about the pipe. The insulated material is formed typically of tar and felt paper. The tar and felt paper serves as an electrical insulator. It blocks the current flow and accordingly resists the weakening of the pipeline wall. The coating process is advantageously performed at the facilities of a fixed yard. The pipe is made in joints and each joint is coated. In the field, the joints are joined together by welding. On larger pipe, an internal weld is placed at the butt connection of adjacent joints and an external bead is also formed. The pipe must be cleaned back to bare the ends. The ends thus are stripped of their tar and felt paper coating. This then leaves the weld and an adjacent few inches exposed after the welding job has been completed. It is therefore necessary to dress the welded portion of the pipe in the field to cover of the weld in the immediate vicinity of the pipeline. Typically, this represents a band or strip of bare metal at the weld which has a width somewhat between four and eight inches.
In times past, the welds have been inspected for weld quality and thereafter they have been tar coated. Many techniques have been used. For instance, it is possible to heat a pot of tar and to mop it onto the surface, thereafter permitting it to cool to serve as the complete patch on the pipeline. By contrast, this is a labor intensive step whereas the method of coating pipe in a pipe yard is highly automated. Because labor costs are extremely high, and moreover, the labor must be performed under the most difficult conditions quite often, it is desirable to improve on the method and manner in which the pipe welds are coated.
The present invention represents an advance over the devices and mechanisms by which pipe welds have been field coated.
The present invention further constitutes an improvement in that it does not require skilled labor. The present invention is advantageous in that it permits a perfect coating to be placed on the pipe. The perfect coating is supplied in sheet form on a spool and is perfect in the sense that it does not have holes, gaps or thin spots in it. When applied, it has a uniform thickness which provides a margin of safety for the coating.